Description will be made on Japanese Utility Model Publication No. 58-32959 as an example of an above described type of mechanical pencil. In this publication gazette, claim 1 is found to describe that “a lead protection device comprising a lead passage tube and a lead holding part both disposed, freely slidably or nonslidably, in the tip of the main body of a mechanical pencil for thin leads, characterized in that the lead protection part is formed by laminating an elastic thin film made of rubber and the like, as integrally molded one piece, on the interior surface of the lead passage tube.” In other words, the integral formation of the elastic thin film on the interior surface of the lead passage tube makes it possible to effectively use even shortened leads.
In this connection, JIS, the Japanese Industrial Standard, specifies the maximum and minimum diameters of the lead, namely, the dispersion range of the lead diameter; for example, a lead of a nominal diameter of 0.5 (mm) is specified to fall within a diameter range from 0.58 mm (maximum diameter) to 0.55 mm (minimum diameter). Accordingly, the inside diameter of the above described lead holding member is designed in conformity with the specified smallest diameter of the leads used to allow a lead having the minimum diameter specified to be held. More specifically, when a lead is delivered, the lead is delivered while the lead is always pressed to expand the lead holding member and is thereby made to advance.
In the above described prior art, the lead holding part sticks fast to the lead passage tube to form one integrally molded piece; accordingly although a certain dispersion range of the lead diameter is allowable, sometimes the leads are not delivered when the maximum diameter leads are used, indicating that the allowable range of the elastic deformation of the elastic thin film, namely, the lead holding member is exceeded.